1. Field of the Invention
The present invention relates to a method and a system of monitoring the deformations of a moving tyre.
In particular, the present invention relates to the possibility of evaluating the deformations of the casing structure of a tyre from within the tyre, for example in order to permit automatic actions, actuated or commanded by the driver, on the vehicle steering and/or control system to modify its behaviour on the basis of measurements made on the tyre.
2. Description of the Related Art
There are known prior art devices for measuring characteristic parameters such as pressure and temperature within a tyre.
Patent application EP 0 887.211 describes a tyre monitoring system comprising a sensor, placed inside the tyre, which is enabled to create an electrical pulse during the passage of the said sensor through the footprint formed by the contact of the tyre with the ground during its rotation. The system described in this patent also comprises means for calculating the ratio of the duration of the said electrical pulse to the duration of one revolution of the tyre, and means for transmitting the said ratio to a processing unit within the vehicle. In particular, the sensor is placed within the tread of the tyre in such a way that the said electrical pulse has a first peak when the sensor enters the footprint area and a second peak when it leaves the footprint area.
According to the teaching of this patent, the ratio between the time elapsed between the two peaks and the period of a complete revolution of the tyre can be used to determine the flattening of the tyre during the running of the vehicle.
This is because the sensor detects the instant of entry into the footprint area and the instant of exit from this area. It is thus possible to measure the length of this area if the angular velocity and the radius of the tyre are known. The length of the footprint area is then related to the flattening of the tyre, which is a critical parameter for the operation of the tyre, particularly in the case of a tyre for heavy motor vehicles.
Patent application EP 0 689.950 describes a different method for monitoring parameters of a tyre such as its pressure and temperature. In particular, this invention uses a self-powered programmable electronic device which is placed on the inner surface of a tyre or on its rim. This device can be used to monitor and store the pressure, temperature, and number of rotations of the tyre, and can comprise an extensometer whose output signal measures the flexion of the inner surface of the tread, or an accelerometer which measures the value of acceleration to which the tread is subjected. Additionally, the device is activated by an external signal at radio frequency and transmits an alarm signal if a predetermined limit of the measured values is exceeded.
There are also known methods for measuring deformations of the tread of a tyre in movement and transmitting them to a receiver located in the vehicle. Patent application WO 93.25400 describes a sensor comprising a resonant circuit which oscillates at a predetermined characteristic frequency, placed inside the tread of a tyre and capable of directly transmitting a signal dependent on the aforesaid deformations. This resonance frequency is modified by the deformations of the tread during movement and the sensor transmits electromagnetic waves proportional to these variations of the resonant frequency. These electromagnetic waves are received by a processing unit connected to a receiver placed inside the vehicle.
U.S. Pat. No. 5,247,831 describes a method for monitoring the behaviour of the footprint area of a tyre during the running of the vehicle in order to optimize the driving of the vehicle. In particular, a piezoelectric sensor consisting of longitudinal strips of piezoresistive rubber is inserted into the tread. This sensor is capable of measuring the deformations of the tread because the strips change their electrical resistance as a function of the said deformations.
There are also known methods and devices for acting on the steering and/or control system of the vehicle, particularly on the devices which regulate the suspension systems of the vehicle, in order to control their behaviour in accordance with information obtained from the axles of the vehicle or from the wheel hubs.
The paper “A method for the evaluation of the lateral stability of vehicles and tires”, produced by Società Pneumatici Pirelli S.p.A. and presented at the International Automobile Tyre Conference, Toronto, Canada, on 22 Oct. 1974, illustrates a way of determining the dynamic behaviour of the vehicle as a function of the forces developed by various types of pneumatic equipment and various conditions of the road surface.
The publication refers primarily to three forces which determine the dynamic behaviour of a tyre, namely the vertical force, the longitudinal force and the lateral force.
The vertical force is the force due to the dynamic load to which the tyre is subjected. The longitudinal force is the force due to the torque applied to the axle of the tyre as a result of the acceleration or braking of the vehicle. The lateral force, which is also present in normal running conditions in rectilinear travel, is the resultant of the forces due to the characteristic angles of the vehicle suspension (camber and toe-in) and to the angular thrust (ply-steer) developed by the layers of inclined cords of the belt structure of the tyre, and the thrust generated by the centrifugal force during side-slip. The sum of the forces developed by the four tyres generates a resultant system applied to the centre of gravity of the vehicle, which balances the inertial forces and determines the trim of the vehicle as a function of the characteristics of the suspension system of each axle. This resultant system is measured by suitable accelerometers positioned on the front and rear axle of the vehicle. The article includes a set of graphs which show the centripetal acceleration or the centripetal force applied to the centre of gravity of the vehicle as a function of the slip angle of the corresponding axle. Graphs plotted for various ground conditions and various types of tyre can be used to establish the behaviour of a vehicle provided with a predetermined set of tyres, and to measure any side-slip in cornering, on either the front or the rear axle.